Wireless connections between different devices have been specified for example in the global “Specification of the Bluetooth system”, v,1.1, Feb. 22, 2001, which is incorporated by reference herein.
The Bluetooth™ specification contains information required to ensure that diverse electronic devices supporting the Bluetooth™ wireless technology can communicate with each other worldwide via short-range radio links. The specification is aimed at providing a standard for low complexity low-cost wireless connections between portable and/or fixed devices, in particular devices which are located within a range of 10 meters to each other. An example for the employment of such a Bluetooth™ short-range radio link is a wireless connection between a laptop and a printer, or between a mobile phone and a cashier for financial transactions.
In some situations a device may want to establish a connection with another device of which it does not know the address, or it may want to determine all devices within its range to which a connection would be possible.
To this end, the specification presents inquiry procedures. In these procedures, a first device that wishes to establish a connection to some other device periodically transmits an inquiry message as broadcast message. The inquiry message has the form of an identity (ID) packet and might also include an indication of the type of devices that are to respond. In case all Bluetooth™ devices in the range of the first device are to be inquired for, a general inquiry access code (GIAC) is included in the inquiry message. In case only Bluetooth™ devices of a specific kind are to be inquired for, one of a number of dedicated inquiry access codes (DIAC) is included in the inquiry message.
All devices that are willing to establish upon an inquiry a connection to some other device, perform a scanning for such inquiry messages in an inquiry scan substate. The length of a scan period in which such a scanning is performed is denoted Tw—inquiry—scan and should lie in a range of 11.25 ms to 2.56 s. The scan interval between two consecutive scans is denoted Tinquiry scan and has a maximum value of 2.56 s. Alternatively, a continuous scanning could be carried out.
When a scanning device detects an inquiry message for which it was scanning, it will respond with a response message, more specifically with a Frequency Hop Synchronization (FHS) packet, carrying parameters of the device. These parameters comprise e.g. the address of the scanning device. However, in order to avoid a conflict when several devices are responding to an inquiry message at the same time, a further protocol is employed, before a response message is allowed to be transmitted. A similar problem may occur with colliding inquiring messages at a scanning device, when several devices are inquiring at the same time for other device to which they might establish a connection. According to this protocol, when a device detects an inquiry message, it generates a random number RAND between 0 and 1023. The device will then go back to sleep or to some other preceding state for the duration of a number of slots corresponding to the generated random number RAND. The duration of such a back-off is thus 0–0.64 s. After the at least RAND time slots, the device will return to the inquiry scan substate and scan again for the same inquiry message detected before. Only when such a further inquiry message is received, the device turns into an inquiry response substate and transmits an FHS packet to the inquiring device, before returning again to the inquiry scan substate.
In between the transmissions of the inquiry message, the inquiring device scans for inquiry response messages. With the information in the received response messages it is then able to establish a connection to the responding device.
A problem with the specified approach for a connection set-up procedure in Bluetooth™ is the long set-up time in the inquiry process, resulting from the mentioned back-off of the scanning devices between the scanning for two inquiry messages. With the currently defined method, the maximum RAND time resulting in a back-off of 1023 time slots is taking at least 25%, and if continuous scanning is considered even 5600%, of the total inquiry time. If a very fast inquiry is desired and therefore the inquiring device or the scanning device are operating all the time, the back-off accounts e.g. for 0.32 s, while 30 ms is the fastest time that could be realized with the inquiry procedure without a back-off. This illustrates also the great impact of the back-off on the absolute inquiry time.
An additional problem with the set-up time arises when two or more devices receive the ID packet. Then it is possible that during the inquiry process the inquiring device receives multiple FSH packets and begins the paging process, which is used for establishing a connection with a specific device, first with a ‘wrong’ device. The ‘right’ pair of devices can never be known for sure beforehand. Considering m—(mobile) commerce with Bluetooth™, it is likely that devices closest to each other form the ‘right’ pair of devices.
In order to reduce the overall time during the set-up of a connection, it has been proposed to reduce the average back-off time during the inquiry procedure by reducing the allowed maximum number for the to be generated RAND to a lower value, e.g. to 256 instead of the current allowed maximum number of 1023. However, a reduction of the maximum number for the RAND increases the probability of collisions. Hence the benefit of reducing the maximum number for the RAND is partly eliminated, because several inquiry rounds may become necessary before a response is received successfully at the inquiring device.